You say it was under roof which in my thinking it was exposed to the weather. Was it ever spun over during that time to keep oil all over the parts. If not you should pull your valve covers and see if the rockers have surface rust.

I was trying to get a 400 Pontiac to turn over a little while back that has been in a barn not started since the mid 80's with no luck. It's rockers are covered with rust as are the cylinder walls so I'm thinking the whole interior is in that condition.

Used both Seafoam and Marvel.
By the way I was using a 4 foot cheater on the breaker bar and loosened the crank bolt without as much as budge of the crank.